Solid-state lighting for general illumination, particularly white LEDs, is being viewed by many as a means of meeting the world's growing needs for lighting, while reducing the associated energy costs. However, white LEDs often suffer from large variations in brightness due to the manufacturing process. Thus, the lighting community has asked the light emitting diode, also referred to herein by the abbreviation LED, industry to reduce the variance of white LEDs to enable luminaire-to-luminaire consistency. (“Lumileds Phosphor Technology Expected to Simplify Binning,” LEDs Magazine web site, Aug. 7, 2007, http://www.ledsmagazine.com/news/4/8/4 (accessed Jun. 3, 2009)).
The main manufacturing steps in a LED manufacturing operation are known. Beginning with a substrate (typically silicon, sapphire or SiC), an epitaxial growth step is followed by lithography, etching and metallization and scribing, bonding, phosphor coating process steps. The combination of scribing, bonding, phosphor coating process steps processes and phosphor costs account for almost 87% of the total cost. Typical LED manufacturing operations require multiple dedicated metal organic chemical vapor deposition (MOCVD) lines to produce red, green and blue LEDs, respectively.
It is noted that the finished product comprises multiple layers, all of which are deposited metal organic chemical vapor deposition techniques (MOCVD). Typical LED growth runs take more than 5 hours, depending upon layer thickness and structure complexity. In a perfect world, the final chip is completely uniform in properties across the surface of the chip. In reality, however, due to imperfections resulting from the metal organic chemical vapor deposition (MOCVD) process and substrate properties such as curvature, LED properties vary significantly across the chip. In turn, this necessitates product separation known as binning, which is done as a function of brightness, color (emission wavelength) and forward voltage. Binning adds huge cost and complexity to the production operation, requiring an additional expensive step and, more importantly, significantly reducing yield.
For example, a typical metal organic chemical vapor deposition (MOCVD) machine will fabricate about 40000 wafers per year, however, roughly 10%, ca. 4000 wafer, will be discarded through the binning process due the unsatisfactory emission wavelengths that fall outside the stringent manufacturing specifications resulting from the variability of metal organic chemical vapor deposition (MOCVD) process. Assuming a cost of $100/wafer, the direct loss from this manufacture variability can be as high as $400000/year for a single metal organic chemical vapor deposition (MOCVD). A large LED manufacturer typically operates 50+ metal organic chemical vapor deposition (MOCVD) machines, thus the potential loss on an annual basis can be greater than $20M. Severe binning criteria can lead to the rejection of >90% of the LEDs.
Generally speaking, there are two basic methods of fabricating white LEDs: the phosphor-conversion method and the discrete color-mixing method. It should be noted that one other method exists, a hybrid method, which is actually a combination of the two aforementioned methods.
The phosphor-conversion method produces white light by coating a blue light-emitting LED die with a phosphor, which is typically yellow. In this case, a portion of the blue light is directly emitted while another portion of the blue light is down-converted to longer wavelengths by the phosphor. The blending of the original blue, short-wavelength radiation with the down-converted longer-wavelength radiation produces white light. The primary means of introducing the phosphor is by dispersing it within an epoxy resin surrounding the blue light-emitting die. In this technique, over half of the photons produced are diverted back to the die. A more efficient technique, the remote phosphor conversion technique has also been developed. This technique involves moving the phosphor away from the die and shaping the optic surrounding the die to efficiently extract the back-scattered photons.
In the discrete color-mixing method, white light is produced by discrete emissions from two or more different colored LED dies. While this method eliminates phosphor conversion losses, this method is viewed as complex and has limited achievable efficacy. Neither technique addresses the problems of wafer variability resulting in binning.
The problems associated with binning are well described in the prior art. For example, U.S. Pat. No. 7,687,816 B2 by E. W. B. Dias of IBM and U.S. Pat. Appl. No. 2010/0067229A1 by A. M Scotch et al. of Osram Sylvania describe the problem that certain types of LEDs, coated with a phosphor to generate white light, exhibit variations in color temperature due to differences in raw material sources, crystal growth, handling, storage conditions for the raw materials, and the other variables that go into the manufacturing process. Therefore, these LEDs need to be binned before use. Such binning processes significantly increase the cost of manufacturing the LEDs.
The process of binning is described very well in U.S. Pat. Appl. No. 2008/0036364A1 by Y. Li and Y. Dong of Internatix. Specifically, they describe the binning procedure in which the electroluminescent characteristics of each LED arrayed on a wafer is measured prior to dicing, after which the individual LEDs are sorted by (a) peak emission wavelength (b) peak intensity of emitted light and (c) forward voltage.
Three broad approaches to ensure consistent color output and overcome binning-related issues are described in the prior art. U.S. Pat. Appl. No. 2008/0036364A1 by Y. Li and Y. Dong of Internatix describes the use of a phosphor combination designed with the ability to correct or self-adjust the chromaticity of emitted light in response to variations in excitation. One phosphor of such ‘smart’ compositions shows a decrease in emission intensity with increasing excitation wavelength while the other shows an increase in emission intensity with increasing excitation wavelength. The problem with this solution is that, in reality, it is challenging to find such ‘smart’ combinations. In addition, this solution does not help with binning that is required due to variation in peak intensity of emitted light or forward voltage differences.
U.S. Pat. Appl. No. 2008/0036364, entitled “Two-phase yellow phosphor with self-adjusting emission wavelength”, provides another method to deal with the binning problem, utilizing phosphors. However, existing rare-earth phosphors are insufficient to achieve high performance warm white light. Rare-earth phosphors induce back-scattering, with losses as high as 60%, and are difficult to mix as different colors have different excitation wavelengths and different temperature sensitivity/degradation characteristics. Narendran, N.; Gu, Y.; Freyssinier-Nova, J. P.; Zhu, Y., Extracting phosphor-scattered photons to improve white LED efficiency. Phys. Stat. Sol. 2005, 202(6) R60-R62.
U.S. Pat. Appl. No. 2007/0285378A1 by M. H. R. Lankhorst et al. describes an LED light source for LCD backlighting that recalibrates itself over time in order to maintain color and brightness uniformity. A solution contains clusters of red, green and blue LEDs, wherein each cluster generates a white light point. Each such cluster has its own controllable driver so brightness of each color cluster is separately controllable. In addition, optical sensors are arranged so that measured color and flux are compared to a stored target and the current to each RGB LED is then automatically adjusted to achieve the target level for each cluster. While this solution is effective, it is simply prohibitive in complexity and cost for most applications, and particularly for applications such as general illumination lighting which are highly cost sensitive.
U.S. Pat. Appl. No. 2007/0215890A1 by G. Harbers et al. of Philips Lumileds Lighting Company describes a white light LED comprising a blue LED over which is affixed a preformed red phosphor platelet and a preformed green phosphor platelet. The LED dies are separately binned in accordance with their light output characteristics and then carefully matched to phosphor platelets of different characteristics such that a consistent white point is generated. This approach is also effective but requires multiple testing steps (the LED dies and each of the phosphor platelets have to be separately characterized) and binning steps. In fact, rather than eliminating binning, it results in increased binning in order to meet the customer requirement of consistent color and intensity output.